DESCRIPTION: The proposed pilot study will evaluate the efficacy of a 30 minute bout of exercise on 28 children ages 9-12 diagnosed with attention deficit hyperactivity disorder and treated with 0.3 mg/kg methylphenidate. The study is motivated by research suggesting that both methylphenidate and exercise modulate calechotamines in the central nervous system (CNS) which may produce similar tranquilizing effects. The effect of methylphenidate on improving attentional and behavioral measures is well documented. With subjects serving as their own control, their performance on the attentional and behavioral measures following administration of methylphenidate will be the standard against which the effect of exercise will be composed. Thus, during three consecutive visits subjects will receive attentional and behavioral testing after receiving no methylphenidate, 0.3 mg/kg alone, and exercise alone. Significant differences on attentional and behavioral measures are hypothesized between the no methylphenidate condition and both the exercise and the 0.3 mg/kg methylphenidate conditions. No significant differences on those measures are hypothesized following exercise and methylphenidate administrations. If exercise proves to have a comparable effect to methylphenidate, further studies would be conducted to determine the duration and amount of effect caused by different amounts and intensities of exercise.